Three dimensional (3D) printing or additive manufacturing is a process in which a 3D digital model is manufactured by the accretion of construction material. The 3D printed object is created by utilizing the computer-aided design (CAD) data of an object through sequential construction of two dimensional (2D) layers or slices that correspond to cross-sections of 3D objects. Stereolithography (SL) is one type of additive manufacturing where a liquid resin is hardened by selective exposure to a radiation to form each 2D layer. The radiation can be in the form of electromagnetic waves or an electron beam. The most commonly applied energy source is ultraviolet, visible or infrared radiation. The liquid photopolymer resin can contain monomers, oligomers, fillers and additives such as photoinitiators, blockers, colorants and other types depending on the targeted properties of the resin.
In the past, the focus of the SL resins has been in the deep range at about 355 nm. These sources work well and there are many formulations for these sources. However, lasers at 355 nm are extremely expensive relying on frequency tripled YVO4 laser crystal technology. Furthermore, DLP projectors are typically unreliable due to UV breakdown the farther away from visible light and are not generally compatible with frequencies below 400 nm. Due to the recent commercialization of Blu-ray laser diodes capable of directly emitting at 405 nm and production of 400 nm to 420 nm direct violet LEDs used in the production of white light bulbs leading to low cost light sources, there has been increased interest in creating SL resins that can function with near UV sources in the about 400 nm to about 420 nm range.
One challenge encountered with SL is the incomplete curing of the printed 3D object including the surface and interior of the printed 3D objects. If the 3D object is cured completely during the 3D printing process, the interlayer adhesion is too weak and the print may fail. In addition, the material may stick to parts of the printing apparatus and not release properly. Hence, it is desirable to cure only in the range of (5% to 99%) and not 100% during the printing process. Afterwards, the uncured resin needs to be removed from the surface and the remaining resin cured to a higher rate. The uncured liquid resin on the surface of the printed 3D objects can be mostly removed by washing with solvents. However, the uncured resin inside the printed 3D object is difficult to remove. Uncured resin inside is undesirable for a few reasons. First, uncured liquid resin leaking from the printed 3D objects may cause health problems to end users because the liquid resin may contain reactive chemicals. Second, the printed 3D objects do not reach optimal mechanical performance because the uncured liquid resin may soften the object. Third, the uncured resin may cause problems in some industrial applications of the objects where high chemical inertia is required.
In order to fully cure the printed objects, the current art is blending excess photoinitiator and post-curing the parts at similar wavelength light used in the printing process. This solution can be problematic because the excess photoinitiator can cause yellowing in transparent printed parts.
Another challenge encountered with SL is creating clear transparent resin using light in the near UV/violet range. Since the near UV/violet light needs to be absorbed to be reacted with the liquid resin, this violet light is normally removed from the spectrum that can pass through resulting in a yellow appearance.
It would be advantageous to use certain types of additives in photopolymer resin formulations to overcome challenges noted above as well as enhancing the performance of the printed objects without causing an increase in the costs.